The present invention relates to a process for reducing the nitrogen oxides present in a lean exhaust gas produced from an internal combustion engine by selective catalytic reduction on a reduction catalyst using ammonia, wherein a fraction of the nitrogen monoxide present in the exhaust gas is oxidized to nitrogen dioxide before the exhaust gas, together with ammonia, is passed over the reduction catalyst.
The process of selective catalytic reduction (SCR=selective catalytic reduction) has been used for a long time in the power station sector to reduce the nitrogen oxides present in the flue gases from a power station. The flue gases from power stations have a net oxidizing effect due to the high proportion of oxygen. The nitrogen oxides present therein can therefore be reduced to nitrogen only with difficulty. This problem was solved by so-called selective catalytic reduction, in which the flue gas is mixed with ammonia and passed over a suitable reduction catalyst at temperatures between 300 and 550° C. The reduction catalysts used are, apart from vanadium and tungsten oxide-containing titanium dioxide catalysts, also ion-exchanged zeolites such as, for example, ZSM-5, mordenite and faujasite. Reduction catalysts which are suitable for selective catalytic reduction are also called SCR catalysts in the following.
Attempts have been made over the last few years also to use selective catalytic reduction for removing nitrogen oxides from the exhaust gases from diesel engines and lean operated gasoline engines. These exhaust gases also contain a high proportion, up to 15 vol. %, of oxygen. About 60 to 95 vol. % of the nitrogen oxides present therein consists of nitrogen monoxide, depending on the operating status of the engine.
Known SCR-catalysts have the disadvantage that their catalytic activity is not high enough for the high space velocities prevailing in the case of car exhaust gas treatment and is subject to rapid ageing. In the case of catalysts containing vanadium oxide, there is a clear loss in activity at temperatures higher than 700° C.
According to DE 198 20 682 A1, it is disclosed that the concentration of nitrogen dioxide in the exhaust gas can be initially raised to 50 vol. % in order to improve the reduction of nitrogen oxides in lean exhaust gases from internal combustion engines. This takes place, according to the cited published application, by oxidizing nitrogen monoxide to nitrogen dioxide in a electrical gas discharge. The exhaust gas treated in this way is then passed over a SCR catalyst while supplying a reducing agent, preferably ammonia. Suitable SCR catalysts which are mentioned are catalysts from the power station denitrogenation sector and also ZSM-5 zeolites exchanged with copper cations.
Also, according to U.S. Pat. No. 5,891,409, it is known that many reduction catalysts have a higher activity for the reduction of nitrogen dioxide than for nitrogen monoxide. Therefore, U.S. Pat. No. 5,891,409 proposes first oxidizing the nitrogen monoxide present in the lean exhaust gas from an internal combustion engine to nitrogen dioxide on an oxidation catalyst and then converting the nitrogen dioxide formed to nitrogen, carbon dioxide and water on a SCR catalyst with the addition of hydrocarbons as a reducing agent. A catalyst consisting of γ-aluminum oxide is mentioned as a preferred reduction catalyst.
WO 99/39809 describes a system for reducing the amount of nitrogen oxides in internal combustion engine exhaust gases, in particular from diesel engines, which consists of an oxidation catalyst which converts at least some of the nitrogen monoxide to nitrogen dioxide, a particle filter, a source of a reducing agent such as ammonia and a SCR catalyst. Using this system, the conversion of nitrogen oxides can clearly be improved. A SCR catalyst based on a V2O5/WO3/TiO2 catalyst is preferably used.
One problem associated with the V2O5/WO3/TiO2 catalysts usually used in the power station sector is the possible emission of volatile, toxic vanadium compounds at elevated exhaust gas temperatures. Replacement of these vanadium-containing catalysts by vanadium-free catalysts, for example catalysts based on zeolite exchanged with transition metals such as iron or copper, does not seem to be suitable due to the known low ageing stability of these catalysts.
It is therefore an object of the present invention is to provide a process for the reduction of the nitrogen oxides present in a lean exhaust gas by selective catalytic reduction, using ammonia, on a reduction catalyst which does not have any, or a substantially reduced as compared with the prior art, emission of vanadium compounds at high exhaust gas temperatures and which is characterized by an outstanding activity and long-term stability of the SCR catalyst.